This paper presents a method and describes an experimental device for determining the steam-water relative permeabilites of unconsolidated porous media. The experimental conditions are as close as possible to those of geothermal reservoirs. The relative permeabilities have been obtained at 180 and 150 degrees C. Their variations versus liquid saturation are quite classical. The air-water relative permeabilities have been measured also at room temperature. The values obtained under these three conditions are almost identical. However, the air-water relative permeability differs slightly from that of steam at 180 and 150 degrees C. We think this discrepancy is acceptable in practice, as it is easier to determine the relative permeabilities for an air-water flow at room temperature than for a steam-water flow at high temperature and pressure.